In applications of high intensity lasers to materials processing, the formation of an ablation plume is of high importance. For wide bandgap insulators (e.g., oxides, halides, nitrides, carbides) irradiated with sub bandgap photon energies, the route to plume formation is not well understood. For example, contrary to metals and semiconductors, inverse bremsstrahlung (IB) is not possible for a wide range of laser intensities on these materials due to insufficient photon and electron densities. We present an alternative path to plume formation on nominally transparent materials. In this paper, we first review the interaction of photo- and thermally-emitted particles from exposure to pulsed laser irradiation of surfaces which include photoelectrons, energetic positive ions, and neutral metal atoms. We establish experimentally that there is overlap in space and time of significant portions of the distributions of these particles in the near surface region. We then present a model for the collected motion of these particles and show that as laser fluence is increased we achieve sufficient densities, overlap, and kinetic energies to result in the onset of plume fluorescence and eventually ionization at fluences far below any IB or catastrophic breakdown process.

We outline results obtained from Schlieren and dye laser resonance absorption imaging of the plume ejected from an aluminum target in a nitrogen atmosphere of 1 bar and 100 mbar by a KrF excimer laser (lambda equals 248 nm, FWHM equals 30 ns). The results show that for relatively low and high laser fluences (14 J/cm² and 36 J/cm²), the plume closely follows the shock wave which is generated by the ablated material pushing against the surrounding gas. Calculations of the evolution of the ambient gas and ablated material show that the temperature and electron density vary greatly depending on the laser fluence and the external gas pressure. We report maximum plasma temperatures of 39888 K and electron densities of 4.2 multiplied by 10²⁶ m^-3 for a laser fluence of 36 J/cm². These results indicate that inverse bremsstrahlung may play a very significant role in how the laser pulse energy is distributed in the plume for high laser fluences.

The objective of this work is to develop a novel process by which topographical changes are produced on a micrometer scale using a pulsed Nd:YLF laser, and to investigate the energy transfer and fluid flow phenomena involved in the process. The surface of thin chromium films is altered through the laser- induced phase transformation and fluid flow. Experimental parametric studies were conducted to correlate the laser parameters with the topography of the laser irradiated surfaces. Experimental and analytical work were also performed to study the transport phenomena involved in the process. A numerical finite element analysis was carried out to simulate the transient field variables. A nanosecond-time resolution, fast photography system was constructed to capture the phase change and the fluid flow occurring at the target surface. Comparison between the numerical and the experimental data helps to understand the mechanisms of the process as well as to develop a controlled surface modification technique.

Laser-micromachining of barium aluminum borosilicate glass, fused silica and stainless steel has been extended down to a pulse duration of 20 fs generated by a Ti:sapphire laser system at a wavelength of 0.8 micrometer. A systematic study shows that, below 100 fs, an enhanced precision and a substantial decrease of the ablation threshold fluence in comparison to pulse laser processing with pulses in the picosecond and nanosecond range could be achieved. The physical mechanism and the technical relevance of this novel microtechnology is discussed.

Results of emission spectroscopy studies of plasmas produced by pulsed (17 ns) laser irradiation of a boron carbide target are presented. Experiments were performed under conditions typical for the pulsed laser deposition of thin doped diamond- like films. Ablation plasma emission spectra in 300 - 600 nm range were recorded both spatially (0.5 and 0.04 mm) and temporally (20 ns) resolved. Plasma electron temperatures (typically 4 - 2.5 eV) were calculated using intensity ratio of CII ion lines (392 and 437 nm). Electron densities (typically 3 - 1 multiplied by 10¹⁹ cm^-3) were calculated using Stark broadened lines of CII (392 and 426.7 nm) and BII (412.2 nm). Self-absorption of the emission from the hot core of the plasma by the surrounding cooler shell of the expanding vapor affected the shape of measured emission profiles. A ratio of carbon to boron species in the plasma of approximately 2 derived with an average plasma temperature of 3 eV indicates a surprisingly low concentration of boron in the plasma plume taking into account the target composition B₄C. The plasma temperature in the hot inner core of the plume may be even higher. With 5 eV however, the mixing ratio (N_C/N_B) becomes approximately 0.37, which is close to the target composition. Taking into account steep density and temperature gradients in the plasma and high velocities of plasma expansion one can expect a dramatic variation of concentration ratio of the two species in time and space. An industrial application of laser plasma emission spectroscopy promises to be an effective tool to control the mixing ratio of species, during laser deposition in order to achieve reproducible film composition and quality.

Exposure of silicon nitride to above-bandgap 6.5-eV photons from an ArF excimer laser drives both the dissociation of silicon-nitrogen bonds and the desorption of nitrogen atoms and/or molecules over a wide fluence range. Crystalline silicon precipitates are also generated on laser exposed surfaces, however, only for fluences exceeding 0.2 J/cm². The rates of nitrogen desorption and the concentration of silicon precipitation were found to depend strongly on laser fluence, rising rapidly above 0.2 J/cm², and saturating at approximately 0.5 J/cm². This saturation was also observed in the thickness of the silicon precipitate layer, which peaked at 35 nm depth for fluences greater than 0.5 J/cm². Such saturation phenomena can be explained by the onset of laser ablation at approximately 0.5 J/cm² fluence which removes material in the laser affected zone. The formation of silicon precipitates is discussed in the context of photochemical reactions that follow band-to-band electronic transitions.

Laser assisted dry etching ablation of (001) InP wafers in a chlorine atmosphere is studied with spatially resolved x-ray photoelectron spectroscopy (XPS). The etching was carried out in a low pressure mixture of chlorine and helium (10% Cl₂ in He). The wafers were exposed to 308 nm pulsed XeCl excimer laser radiation with several values of fluence near the ablation threshold of InP. It was found that, at room temperature, the applied etch mixture does not spontaneously react with the wafer. It was also found that laser irradiation at a fluence less than the ablation threshold of InP stimulates a chemical reaction between the chlorine and the wafer, forming In-Cl compounds. At the same time, the irradiation removes the reaction products.

Epitaxial Si films are grown on Si substrates by synchrotron radiation-excited gas-source molecular beam epitaxy (MBE) using disilane. It is demonstrated that the epitaxial temperature is lowered to 40 degrees Celsius. Selective epitaxial growth between Si/SiO₂ substrate can be achieved irrespective of growth time at temperatures above 700 degrees Celsius. For the B doping using disilane/decaborane, it is confirmed that SR irradiation significantly decreases the doping temperatue (80 degrees Celsius) and electrical activation rate.

Nonlinear optical materials comprising metal nanocrystallites embedded in linear and nonlinear dielectrics are of wide current interest for use in all-optical switching devices. We have investigated several ways in which laser- and ion-beam processing can be used to create vertically and/or laterally patterned nanostructured composite materials. Pulsed laser deposition using both metal and dielectric targets can be used to create layered structures in which some layers contain quantum dots as a nonlinear element. Ion beams can also be used to induce the formation of deep waveguides in soda-lime glass subjected first to Ag ion exchange. When these Ag quantum-dot composites are irradiated by high-intensity laser light, a photochemical reaction generates Ag₂O nanoclusters, changing the sign of the nonlinear index of refraction. This phenomenon offers unusual possibilities for spatially modulating a nonlinear waveguide with very high lateral resolution. Finally, we consider the use of lasers in conjunction with laser- or particle-beam-created surface defects to serve as distributed nucleation sites for quantum- dot growth. Atomic-force microscopy on planar versus offcut surfaces shows that substrate orientation, temperature and deposition pressure can be used to control the size distribution and two-dimensional growth pattern of Au nanoclusters on strontium titanate substrates.

The direct-write laser machining technique has been used to process a lithium-alumosilicate glass (Foturan^TM) for an application which requires 3D patterned microstructures. Using two UV laser wavelengths (248 nm and 355 nm), microcavities and microstructures have been fabricated for the development of microthrusters for attitude and orbit control of a 1 kg class (10 cm diameter) nanosatellite. In addition, experiments have been conducted to define the processing window for the laser patterning technique. The results include a measure of the change in Foturan strength after a required program bake cycle plus HF etching rates as a function of the laser repetition rate for the two UV wavelengths.

Laser-induced quantum well intermixing (laser-QWI) and ion implantation-induced quantum well intermixing (II-QWI) techniques have been studied to selectively modify the optical properties of GaInAsP/InP laser microstructures. Following the annealing with a cw Nd:YAG laser, a blue shift in the quantum well photoluminescence of up to 124 nm was observed for samples annealed up to 4 min. A comparison of the laser annealing results with those of II-QWI, which were obtained for the same GaInAsP/InP microstructure, indicates that laser- QWI yields material with comparable, or better optical properties. The one-step processing used in the laser-QWI approach makes it an attractive alternative in fabricating photonic integrated circuits at low cost.

Use of suitable laser systems for photovoltaic devices production has received great attention in recent years, and lasers today's applications range from panel patterning to advanced doping. Mainly when crystalline and polycrystalline silicon are concerned, it seems that lasers can be usefully used to get high efficiency devices in industrial production plants. In this paper, we describe our studies on advanced laser applications for silicon solar cells processing: grooving, texturing, and doping experiments are reported, both on simple structures and on devices, with efficiency values ranging from 15.0% to 16.5%, according to the particular design used. Future developments on schedule in our laboratory, devised to increase cells efficiency and to facilitate lasers use in industrial plants are briefly overviewed.

A new class of laser-based tools now permits the ultra-high- speed direct etching of substrate or circuit layers, and direct deposition of metal interconnect. These new methods have important applications in design debug/failure analysis and in micromachining. The deposition capability enables the rapid reconfigurration of an integrated circuit (IC) or multichip module (MCM) in real-time from a live color image. Metal lines are added or deleted through a graphical user interface with operations occurring on an actual part rather than a data base. Etching tools provide a means for locally thinning silicon integrated circuits, an essential step for testing of flip-chip circuits. An intricate microelectromechanical system (MEMS) can be carved directly into silicon from a 3-D data file without the need for masks or, alternatively, it may be trimmed to optimal performance as it oscillates under the laser focus. With laser microchemical technology, microelectronic parts can be locally machined without introducing process stress or contamination; micron- thickness metal lines are laid down in a one-step vapor phase deposition at 200 micrometers per second writing speed. Rapid deposition combined with the superior metallurgy of the laser interconnect, translates into writing with a conductance per unit writing time of 1000 to 10,000 times the rate of a focused ion beam. Silicon is etched at greater than 1000,000 cubic microns per second while retaining an average surface roughness of several hundred angstroms.

The purpose of the work is to investigate a photoluminescence of GaAs-photocathodes at various stages of manufacturing depending on illumination method and spectral range of radiation, but also to obtain 2-dimensional spatial distributions of a photoluminescence intensity, to compare them with reference and to detect features of indicated distributions. Besides with the purpose of detection and extraction such typical defects as internal cracks, pile-up of dislocations and swirl-defects some algorithms of the obtained images processing were developed and tentatively tested.

IBM introduced the first commercial high-end mainframe computer system incorporating laser ablation technology in 1991. This milestone was the culmination of nearly a decade of scientific, engineering, and manufacturing effort. Extensive research and development on 308 nm laser ablation of polyimide lead to the first IBM prototype ablation tool in 1987 for the production of via-holes in thin film packaging structures. This prototype, similar to step and repeat photolithography systems, evolved into full-scale manufacturing tools which utilize sophisticated beam shaping, beam homogenizing, and projection optics. But the maturity of this technology belies the fact that the scientific understanding of the laser ablation process is still far from complete. This paper briefly reviews the engineering and scientific accomplishments, both within and external to IBM, that lead to the commercial utilization of the laser ablation process. Current technical tissues are discussed, in addition to alternative IBM applications of polyimide ablation. The paper concludes by discussing the relative merits of excimer vs. solid-state lasers, and how each may impact future manufacturing technology.

Multiparametric study of A1N ceramic ablation by high intensity (I less than or equal to 10¹³ W/cm²) nano and picosecond pulses of Nd:YAP laser have been performed. Ablation rates, surface morphology and element content, reflectivity and absorptivity of ceramic plates prior and after pulsed irradiation as well as high temperature material reflectivity have been measured. It is shown that high etch rate and good quality A1N ceramic microstructuring (cutting, hole drilling, pocket formation) can be obtained for the first, second and fourth laser harmonics if irradiation conditions are properly chosen. It was found that possibility to effectively process material, which initially weakly absorbs laser beam (first and second harmonic), it is determined by radiation and plasma induced surface modification. Comparison with other types of ceramics, such as Al₂O₃, Si₃N₄, SiC, is also made.

We describe the rapid fabrication of binary, multilevel, and blazed diffractive optical elements, using a tabletop excimer laser micromachining workstation and optical surface profiler. Functional DOEs are produced in minutes to hours in polymer films. We also show that laser-machined polymer DOEs can be transferred to fused silica by reactive ion etching.

Laser ablation was applied for the first time to define a rib waveguide on silica-based planar waveguides. The strong linear absorption of 157-nm radiation was key for producing smooth walls without surface microcracks or significant debris. The laser-formed rib guides were single-mode at 0.635-micrometer wavelength and had optical losses of approximately 4 dB/cm as a result of an approximately 40 nm (rms) surface roughness. Simple refinements to the laser etching procedure and extensions to 1.55-micrometer telecommunication wavelengths promise commercially attractive losses (less than or equal to 1 dB/cm) for application in processing optical integrated circuits.

Development of advanced engineering materials, such as ceramics and their composites, CVD silicon carbide, silicon nitride, CVD diamond, etc.; has created many opportunities for their use in a number of applications. These materials offer attractive properties, including superior hardness, strength, wear and corrosion resistance, and electronic properties. However, conventional machining processes are inadequate to fabricate multi-dimensional, precision components from these materials. Lately, lasers have been shown to be very effective in machining these hard materials. In this paper, recent results of our work on depth controlled precision machining of commercially available ceramics using a diode pumped, pulsed Nd:YAG laser are presented. The effects of laser processing parameters: percentage spot overlap, machining speed, and number of layers removed on the depth of the machined groove and surface roughness are discussed.

Micromachining requires very precise tools with high resolution. Lasers emitting electromagnetic radiation in the ultraviolet (UV) region offer focussed spot sizes less than one micrometer. Wavelengths in the range of lambda equals 200 to 280 nm are called deep ultraviolet (DUV) and ensure minimal resolution for actual and future application in microelectronics. Conventional DUV-lasers are the well developed excimer-lasers and frequency converted systems such as Nd:YLF, Nd:YAG and Ar+-lasers. DUV-lasers can be used for submicron single pulse machining up to processing of complex surfaces for the great variety of microelectronic components. The process efficiency is determined not only by the choice of the laser source itself, but also by the system technology such as optical elements for beam shaping and guidance or workpiece handling. Besides the system technology, the choice of an appropriate laser and handling system ensures an efficient processing and repair of microelectronic components. The presented examples cover the direct writing of conductive layers on ceramic material in combination with electroplating, which offers ways of rapid prototyping printed circuit boards (PCB). Furthermore, the repair of expensive electronic products is of growing interest. Examples are the repair of photolithography masks. An overview of further opportunities using DUV-lasers is given by 3D structuring of glass and ceramics.

A diode-pumped laser system is described operating at up to 1 kHz pulse repetition frequency. The system can be used at the wavelengths 1064 nm, 532 nm, 355 nm or 266 nm. Up to 10 W at 1064 nm and up to 6 W at 532 nm are generated. More than 4 W of average power is available at 355 nm from frequency tripling modules, still in a diffraction limited beam. Frequency quadrupling of the laser has delivered more than 2 W average power at 266 nm. The pulse repetition frequency and output pulse energy are continuously adjustable without changes in the other laser parameters. The high energy and the short duration of the laser pulses of 10 - 12 ns in combination with the high beam quality allows one to achieve power densities exceeding 100 GW/cm² with simple focusing optics. Examples of applications in micromachining are given, and current limitations as well as future trends are discussed.

In this work, Si_1-yC_y and Si_1-x-yGe_xC_y alloy layers were grown by multiple energy ion implantation of Ge and C into single crystal Si followed by pulsed excimer laser induced epitaxy. The properties of the alloy layers obtained by this technique, in terms of film crystallinity, Ge and C redistribution an substitutional incorporation, strain formation and relaxation, SiC precipitation, were demonstrated to depend strongly both on ion implantation and laser processing conditions. The growing of pseudomorphic epitaxial layers, from group IV semiconductor alloys, using the very high energy and large area beam (up to 1 J/cm² per pulse over 40 cm²) excimer laser developed by SOPRA, for mass production is reported for the first time.

Lasers have been used in industry for well over 20 years and are currently found in many application areas including welding, cutting, drilling, surface modifying and material removal. The general industry trend has been toward high power lasers for macro applications, frequently involving metal working. More recently a major shift in emphasis has been made by some manufacturers to more fully investigate the feasibility of using lasers for the purposes of what we describe as 'micromachining.' In general this involves working with material of limited bulk and thickness (less than 1 mm) and where feature sizes are also on the order of several mm maximum. This talk will briefly describe the advantages and disadvantages of several laser types and the components of turn-key, laser based micromachining systems. Many successful applications are presented and discussed including material processing of ceramics, CVD, diamond, plastics and thin metals, used in industries such as micromedical devices and microelectronic manufacturing.

Pulsed laser deposition (PLD) has been used to deposit high quality thin films of Ni₈₁Fe₁₉/Au/YBa₂Cu₃O_{7- (delta} ) onto (100) oriented substrates of MgO and SrTiO₃ for the purpose of fabricating a novel high temperature superconducting three terminal device. The ferromagnet-normal metal-superconductor (F-N-S) structure is currently being investigated to determine the effect of the injection of a spin-polarized current on the order parameter of a high temperature superconducting thin film. High quality films with sharp interfaces, free of defects, are required in order to maximize the spin-injection effect. The surface morphology and transport properties of the YBa₂Cu₃O_7-(delta ) films have been investigated using scanning electron microscopy and ac susceptibility measurements, respectively, as a function of increasing laser fluence. Deposition at 2.0 - 2.4 J/cm², 790 degrees Celsius and 320 m Torr O₂ produces films with a sharp superconducting transition and a smooth surface. The growth of Au on YBCO under different PLD conditions has been observed by atomic force microscopy. Surface clustering of Au occurs at elevated temperatures and is attributed to increased surface mobility. The presence or absence of a background gas influences the cluster size. These results are discussed within the framework of the role of excess energy of PLD adatoms with changing laser fluence and background gas.

High quality thin films of Sr_xBa_(1-x)TiO₃ are currently being grown using pulsed laser deposition (PLD). These films are being used for the construction of frequency tunable microwave electronic devices. In particular, a low phase noise, voltage controlled oscillator (1.5 - 2.5 GHz) is currently being developed. Single phase and oriented Sr_xBa_(1-x)TiO₃ films have been deposited by PLD onto (100) LaAlO₃ and MgO and single crystal Ag films. The dielectric properties of these films has been measured at 1 MHz and between 1 and 20 GHz. A 75% change in the capacitance can be achieved using a 40 V bias across a 5 micrometer interdigital capacitor gap (80 kV/cm). The dissipation factor (measured at 1 MHz) depends on film composition and temperature. Dielectric loss measurement at 1 - 20 GHz have shown a dielectric loss tangent as small as 1.25 multiplied by 10^-2.

It has been shown that in bulk ceramic form, the barium to strontium ratio in barium strontium titanium oxide (Ba_{1- x}Sr_xTiO₃, BSTO) affects the voltage tunability and electronic dissipation factor in an inverse fashion; increasing the strontium content reduces the dissipation factor at the expense of lower voltage tunability. However, the oxide composites of BSTO developed at the Army Research Laboratory still maintain low electronic loss factors for all compositions examined. The intent of this study is to determine whether such effects can be observed in the thin film form of the oxide composites. The pulsed laser deposition (PLD) method has been used to deposit the thin films. The different compositions of the compound (with 1 wt% of the oxide additive) chosen were: Ba_0.3Sr_0.7TiO₃, Ba_0.4Sr_0.6TiO₃, Ba_0.5Sr_0.5TiO₃, Ba_0.6Sr_0.4TiO₃, and Ba_0.7Sr_0.3TiO₃. The electronic properties investigated in this study were the dielectric constant and the voltage tunability. The morphology of the thin films were examined using the atomic force microscopy. Fourier transform Raman spectroscopy was also utilized for optical characterization of the thin films. The electronic and optical properties of the thin films and the bulk ceramics were compared. The results of these investigations are discussed.

Pulsed laser deposition has been used to deposit multilayer heterostructures consisting of high temperature superconductor (HTS) and ferroelectric layers for tunable microwave device applications. The dielectric nonlinearity and loss of the ferroelectric thin films are the key material parameters determining the performance of the tunable devices and hence the feasibility of this technology. In this paper, I summarize the current understanding of these issues and outline the strategies to study these problems.

The results of pulsed laser deposition of Ti-doped and undoped sapphire ((alpha) -Al₂O₃) thin films are presented. Three types of doped targets are utilized -- monocrystalline and two different ceramics synthesized in vacuum and H₂ ambient, respectively. The doping procedure is related with the potential application of the Ti:sapphire thin films as planar tunable laser media. The influence of the target's type upon the film quality is studied. The films are characterized by XRD, RHEED, and SEM analyses. The optical emission measurements of the laser induced plasma plume are performed.

One proposes the method of neutral atom beam formation due to effect of neutral atoms long-time localization in the minimums of potentials of intense standing wave of resonant light with simultaneous influence of cooling radiation. For fields of complicated configurati on the resonant light pressure force and the momentum diffusion tensor of two-level atom ensemble are presented. Mathematical modeling is based on the solving of equations for their spatial terms. One shows that with cooling time about 2 - 3 ms and with manipulation of intensity of co-propagated light wave, the arbitrary given cross-section atom density distribution with the contrast up to 1000 can be produced. One demonstrates that size of localization region can be essentially decreased by sweeping of cooling light carrier frequency in the range about 2 - 3 widths of moving atoms spectral transition lines. Low temperature cooling of atoms in the beam and 'soft' output from interaction region allow us to produce the well-manipulated practically aberration-off process. In this case transverse compression of image in cross-section of atomic beam can reach to 10⁶ with given level of contrast. Calculations shows that at modern level of laser technique the spatial resolution of such process can reach to 30 - 50 A. The possibility of using of given method for high-resolution development and the analyses of micro-object surfaces is discussed.

The possibility of using the pulsed laser deposition technique to produce garnet (Y₃Fe₅O₁₂) and ferrite (Mn_{1- x}Zn_xFe₂O₄) thin films of high quality on different substrates (poly-Al₂O₃, poly-AlN, sapphire) is explored. The properties of thin films prepared by excimer laser ablation of stoichiometric non-sintered (Y₃Fe₅O₁₂) and sintered (Mn_1-xZn_xFe₂O₄) ceramic targets are performed. The stoichiometrical transfer from targets to substrates, as well as synthesis of YIG phase in the case of ablation of non-sintered Y₃Fe₅O₁₂ target is achieved. The magnetic measurements in two in-plane perpendicular each to other directions are carried out. The hysteresis loops observed show evidence of anisotropy which depends on the crystal structure of substrates. The typical values of coercive force and saturation magnetization measured appear to be comparable to those of more thick films. The films are characterized by VSM and XPS analyses.

Today multichip modules (MCMs) have found applications in a variety of fields including computers, telecommunication, automotive industry, and medical diagnosis devices. Lasers are being used as a processing tool for fabricating high density multilevel thin film packages for MCMs. The two most commonly practiced laser processes for multilevel thin film packaging are, laser via ablation and laser based circuit repair processes. Laser via ablation is used for creating via holes in polyimide to provide vertical connection between two adjacent layers of multilevel thin film. It is a dry, precise, and highly robust patterning technology available today in packaging industry. The three major aspects of via ablation technology are ablation process, mask technology, and tooling. IBM has pioneered the laser via ablation technology and has developed all three aspects to use it as a primary technology for via formation for thin film packages. Laser based circuit repair processes have also been developed to a mature state where they are being used on a routine basis to repair circuits in multilevel thin film packages. The need for repair of circuit arises for variety of reasons including, contamination, yield improvement, to accommodate engineering changes or to correct design errors. The commonly practiced laser based repair processes are deleting metal shorts using a laser, depositing metal using laser chemical vapor deposition technique, and stitching metal lines using laser sonic bonding technique.